Virtual electrode imaging chamber

ABSTRACT

An imaging chamber for an x-ray system using a dielectric or other suitable receptor of a latent electrostatic image in the gap between electrodes. A chamber with substantially planar electrodes, either flat or slightly cylindrical, with each electrode having a dielectric substrate with low conductivity surface at the gap with a plurality of spaced conductors thereon for connection to a voltage divider to produce electrostatic potentials at the gap surfaces the same as the electrostatic potentials of concentric spherical metal electrodes.

United States Patent Proudian et al.

[ Nov. 25, 1975 VIRTUAL ELECTRODE IMAGING Primary Examiner-Davis L. Willis CHAMBER Attorney, Agent, or Firm-Harris, Kern, Wallen & [75] Inventors: Andrew P. Proudian, Chatsworth; Tmsley Murray S. Welkowsky, Sherman Oaks; Steven A. Wright, Playa del [57] ABSTRACT of calm An imaging chamber for an x-ray system using a di [73] Assignee: Xonics, lnc., Van Nuys, Calif, electric or other suitable receptor of a latent electro static image in the gap between electrodes, A chamber [22] 1974 with substantially planar electrodes, either flat or [2i] Appl. No.: 530,057 slightly cylindrical, with each electrode having a dielectric substrate with low conductivity surface at the 52 U.S. c1. 250/315 A gap a plural'tylof f j therelonfor mt Cl-z 841M connection to a v0 tage 1v1 er 0 pro uce eec ron Id "256/ A 374 stat1c potentials at the gap surfaces the same as the e o earc electrostatic potentials of concentric spherical metal l t d [56] References Cited 6 ec r0 es UNITED STATES PATENTS 3,859,529 H1975 Proudian et al. .1 250/3l5 A 3,879,6l0 4/1975 Baker 1, 250/315 A 24 Clams 9 Drawmg F'gures 3,883,740 5/l975 Pr0udian.... 250/3l5 A qf/O l t/-l J- I i I 22 i /z /L I I) l A 2 mafiaaziz zazaazazzwea 70 POWER I\\\\\\\A\\\YI\\\\\\\\Y V 5UPPL/E'5 I I ///f 30 US. Patent N0v.25, 1975 Sheetl0f2 3,922,547

42 Era. 3.

VOL TA 65 5UPPL Y VOLTAGE ax/PPM VOL TA GE 5 U PPL Y VIRTUAL ELECTRODE IMAGING CHAMBER This invention relates to electronradiography and in particular, to a new and improved imaging chamber. In electronradiography or ionography, an x-ray opaque gas is used between two electrodes in an imaging chamber to produce a photoelectric current within the chamber, which current is collected on a dielectric sheet placed on one or the other of the electrodes, resulting in a latent electrostatic image. The latent image is then made visible by xerographic techniques.

An x-ray source is used to create primary photoelectrons in a gas in the gap between the electrodes of the imaging chamber. Typical imaging chambers have planar or cylindrical electrodes and the oblique incidence of the incoming x-ray produces geometric unsharpness in the resultant image. One solution to this problem is set out in the copending applications Ser. No. 388,212, now U.S. Pat. No. 3,859,529, filed Aug. 14, I973, entitled Ionography Imaging Chamber and Ser. No. 388,262, now U.S. Pat. No. 3,883,740 filed Aug. 14, I973, entitled Ionography Imaging Chamber for Variable Distance X-ray Source, both copending applications being assigned to the same assignee as the present application. In the imaging chamber of the copending applications, the electrodes are constructed in such a manner that the potential variations at the electrode surfaces correspond to that of concentric spherical equipotential in the imaging gap. In the preferred embodiment, a layer of finite and variable conductivity material is used for each electrode, with the potential applied between the center and periphery of the elec' trodes. Reference may be had to the copending applications for a complete discussion of the problem and the disclosed solution.

However, these copending applications call for control of the physical characteristics of a material, such as conductivity and/or thickness which must vary in a prescribed manner over a required range, all of which is difficult and complex. Accordingly, it is an object of the present invention to provide a new and improved imaging chamber electrode construction which is simple in design and inexpensive and easy to manufacture while achieving the desired electrostatic potentials at the gap.

The present invention contemplates the use of a dielectric substrate with a low conductivity layer at the gap surface of each electrode, which layers may be of uniform thickness and resistivity, with spaced conductors on the substrates and connected to appropriate voltage sources for producing the desired electric field. Other objects, advantages, features and results of the invention will more fully appear in the course of the following description.

In the drawing:

FIG. 1 is a diagramatic illustration of an x-ray system with an imaging chamber incorporating the presently preferred embodiment of the invention;

FIG. 2 is a plan view of one of the electrodes of the chamber of FIG. I;

FIG. 3 is a schematic of the electrical connections for the chamber of FIG. I;

FiG. 4 is a perspective view ofa cylindrical electrode, illustrating an alternative embodiment of the invention;

FIG. 5 is a sectional view of an alternative electrode construction;

FIG. 6 is a plan view of another alternative electrode construction;

FIG. 7 is a sectional view taken along the line 77 of FIG. 6 and including a second electrode and an electrical schematic similar to that of FIG. 3;

FIG. 8 is a perspective view illustrating the use of the round electrode of FIG. 6 in a rectangular configura tion; and

FIG. 9 is a view similar to that of FIG. 4 illustrating a cylindrical embodiment of the electrode of FIG. 6.

The system as illustrated in FIG. 1 includes an x-ray source 10 positioned for directing radiation to an object II which may rest on a table 12. An imaging chamber 13 carrying a dielectric receptor sheet I4 may be positioned below the table, with x rays from the source passing through the object 11 and into the gas-filled gap 15 of the imaging chamber I3. The design of the imaging chamber itself is not a feature of the present invention and various of the known imaging chambers may be utilized.

The imaging chamber may comprise a housing 20 with cover 21 and electrodes 22, 23 mounted therein, defining the gap 15 therebetween. Gas may be introduced into the chamber via line 28, and the electrodes are connected to the power supply via cables 29, 30.

A preferred embodiment for the electrode 22 is shown in FIG. 2 and comprises a dielectric sheet substrate 31 with a plurality of concentric conductors Rl R7 on one surface of the sheet and a layer 32 of low conductivity material over the conductors. Since most x-ray pictures are rectangular or square, arcuate circular segments are utilized in the corners. While a seven conductor electrode is illustrated in FIG. 2, various numbers of conductors may be utilized depending upon the size of the imaage and on the accuracy of simulation of the sperical potential desired. In one embodiment, a [4 inch by I7 inch image is obtained with electrodes I7 inches by 20 inches and having 15 concentric conductors.

Means are provided for connecting the conductors RI R7 to a power supply and typically may comprise additional conductors 35 on the opposite surface of the substrate and interconnections 36 through the substrate between the conductors R1 R7 and the conductors 35, with the conductors 35 being connected to the cable 29 for connection to the power supply as illus trated in FIG. 3. The electrode 23 may be constructed in the same manner as the electrode 22, with the conductors RI R7 at the gap surface.

A voltage supply 37 is connected across a voltage divider 38 and another voltage supply 39 is connected across another voltage divider 40. The points on the voltage divider 38 are connected to the conductors R1 R7 of electrode 22 by cable 29, and the points on the voltage divider 40 are connected to the conductors Rl R7 of electrode 23 by cable 30. Another voltage supply 42 is connected across the electrodes, preferably by being connected to the corresponding conductors on the gap surfaces of the two opposing electrodes, as illustrated in FIG. 3.

If desired, a variable resistor 43 may be connected in parallel with the electrode 22 and a variable resistor 44 may be connected in parallel with the electrode 23. Either or both of the resistors 43, 44 may be used to compensate for changes in the distance between the x-ray source It) and the imaging chamber I3, operating in the manner described in detail in copending application Ser. No. 388,262, filed Aug. M, 1973 and assigned to the same assignee as the present application.

The imaging chamber of the present invention provides the desired spherical equipotentials on the electrodes. with the concentric conductors defining radial locations at which the voltages are applied. A close approximation to the desired condition is achieved by applying the voltages at the spaced locations along the surfaces of the electrodes, with the approximate potential configuration approaching the ideal configuration as the number of conductors on the electrode surfaces is increased. The maximum resolution required in a particular instrument dictates the number of conductor locations on the electrodes.

The electrode which is on the x-ray source side of the gap desirably should not contain any x-ray absorbing material of a localized nature within the detection capability of the imaging chamber. since this material would appear on the resulting picture. Any uniformly absorbing material over the entire image area would not appear on the final image, but would reduce the net efficiency of the instrument. Therefore, the total x-ray absorption of the electrode 22 and also of the housing cover 21 should be minimized. This problem does not exist with the lower electrode where x-ray absorption is not a factor.

In the preferred embodiment illustrated, the substrate is a polyester sheet typically having a thickness in the range of 10 to 14 mils, this material having sufficient dielectric strength and mechanical rigidity. The conductors may be applied to the electrode by laminating thin aluminum sheet on both surfaces and then producing the desired conductor configuration by photoetching. Typically the aluminum sheet can be onefourth mil thick. The width of the conductors on the electrode 22 should be minimized to avoid the conductors appearing in the final image, and typically the con ductors are in the order of l mils wide. Conductor width is not a problem on the electrode 23, but ordinarily the two electrodes will be identical for ease in manufacturing. The interconnections between the conductors on opposite sides of the electrode should also be nonimaging and typically may be a carbon adhesive such as a mixture of lampblack and a polyester adhesive. Of course, other materials and sizes may be used as desired.

The material used for the low conductivity layer 32 on the substrate 31 may be a carbon impregnated epoxy which is stable and uniform in composition. The low conductivity layer may be made separately and at tached to the polyester substrate by a thin film of adhesive or may be applied in fluid form and cured on the substrate.

The resistivity of the low conductivity layer at the gap surface of the electrode preferably is in the range of about 10 to ohms per square. The upper resistivity limit (the lower electrode current limit) is dictated by the desire to have the electrode current greater than the gap current so that fields created by the gap current do not distort the desired spherical field created by the electrodes. The lower resistivity limit (the upper current limit) is dictated by the size of the power supplies needed to provide the currents, with higher currents requiring larger power supplies. In a typical instrument, each of the three power supplies may provide outputs in the order of kilovolts, with the parameters chosen such that the current in a voltage divider preferably is in the order of 10 times greater than the current in the 4 connected electrode so as to prevent any variation in divider output due to small changes in conductivity of the electrode between adjacent rings due to variations in manufacturing conditions.

An alternative embodiment of the electrode 22 is shown in H0. 5, where the low conductivity layer 32 is formed as two layers 32a and 32b. Layer 32a is on the substrate 31 over the conductors R1 R7, and layer 32b is on layer 32a. Layer 32a is thinner and of higher conductivity than layer 32b. The receptor 14 is positioned at the layer 32b which functions to spread the voltage from the conductors and thereby spread or diffuse the conductor image which might result from the conductors being between the receptor and the x-ray source. This problem is not encountered with the electrode 23 and ordinarily there would be no need for the double layer construction for electrode 23.

The layer 32b desirably is about at least twice as thick as the layer 32a and preferably in the range of 2 to 4 times as thick. Typically the layer 32a is in the order of 0.010 to 0.020 inches thick and the layer 32b is in the order of 0.040 to 0.060 inches thick.

The layer 32b desirably has a resistivity about at least ten times that of the layer 320 and preferably in the range of l0 to times. Typically the layer 32a has a resistivity of about 10 to 10 ohms per square and the layer 32b has a resistivity of about l0 to 10 ohms per square.

The preferred embodiment of the imaging chamber described above has utilized electrodes with flat parallel gap surfaces. The present invention is equally applicable to electrodes with concentric cylindrical gap surfaces. The relation between the flat parallel gap surfaces and concentric cylindrical gap surfaces is discussed in the aforesaid copending application Ser. No. 388,212. A typical cylindrical electrode 23 incorporating the present invention is illustrated in FIG. 4, with parallel conductors 1 7 on the gap surface side of the substrate 31' and connected to conductors 35' on the opposite surface by interconnections 36', with low conductivity layer 32 and with cable 30 providing for connection of the electrode to the voltage divider 40. The lower electrode 23' would be concave when viewed from a gap surface while the upper electrode (not shown) will be convex when viewed from the gap surface. The electrodes may be manufactured in a flat position and formed to the slightly cylindrical configuration by the housing of the imaging chamber in which they are mounted. In a typical instrument with the distance from the x-ray source to the imaging chamber in the order of two meters, the edge of a fourteen inch wide cylindrical electrode will be about a millimeter out of plane. Hence the expression substantially planar covers both electrodes with both flat parallel gap surfaces and electrodes with concentric cylindrical gap surfaces.

Another alternative electrode construction is shown in FIGS. 6 and '7. Electrodes 52, 53 correspond to electrodes 22, 23 of FIG. 1. However, the electrodes are formed with a plurality of concentric sections 55. All the sections 55 are oflow conductivity material, but adjacent sections have different conductivity, with the section conductivity decreasing from the center to the edge of the electrode. The conductivity for each section desirably is substantially uniform throughout the section. The electrodes 52, 53 may be of carbon impregnated epoxy in the order of l0 to 14 mils thick, with the resistivity varying from 10 ohms per square at the center to ohms per square at the edge. Typically a 17 inch diameter electrode may have concentric sections.

The electrodes of FIGS. 6 and 7 do not require the voltage dividers 38, 40 used with the electrodes of FIGS. 1 and 2 having the discrete conductors. Otherwise, the electrical circuitry is substantially the same, utilizing the power supplied 37, 39 and 42. A piece of aluminum foil 57 may be affixed at the center of the electrode and a ring of aluminum foil 58 may be affixed at the rim of the electrode for making the electrical connections. The shunting resistors 43, 44 may be used if desired.

The imaging chambers presently in use call for rectangular receptors and rectangular electrodes. The circular electrodes of FIGS. 6 and 7 are readily formed into the rectangular configuration, as shown in FIG. 8. The electrode 53 is laid onto a dielectric substrate 54, typically a 14 mil thick mylar sheet, of the desired rectangular shape and size. The overhanging edges 58 of the electrode are folded over the edge of the substrate 54 and affixed to the reverse side, preferably with a conducting shield, such as a 1 mil aluminum foil, positioned between the edges 58 and the substrate 54.

As with the electrode configuration of FIGS. 1 and 2, the electrode configuration of FIGS. 6 and 7 can be provided in a cylindrical shape. A cylindrical electrode 53' with parallel sections 55' is shown in FIG. 9. The sections 55' are of low and substantially uniform conductivity, with the conductivity decreasing from the center section to each edge. The various parameters set out in the description of the preceding electrodes are applicable to the cylindrical electrode of FIG. 9.

The presently preferred method of making the sectional electrode of FIGS. 69 is to use a substrate with a plurality of conductors thereon, such as the substrate 31 with the conductors R1 R7. The low conductivity material is applied over the conductors in the uncured condition. Each section is then cured separately by applying a voltage source across the conductors which define the section. The applied voltage heats the section and the curing time and temperature is selected for each section to produce a cured material having the de sired conductivity.

We claim:

1. In an imaging chamber for an x-ray system, the combination of: I

first and second substantially planar electrodes;

means for mounting said electrodes in the chamber in spaced relation defining a gap therebetween;

each of said electrodes having an electrical insulating substrate with a low conductivity surface at said gap and means defining a plurality of spaced locations along the surface; means for connecting a first voltage source to said first electrode providing defined voltages between said spaced locations of said first electrode;

means for connecting a second voltage source to said second electrode providing defined voltages between said spaced locations of said second electrode; and

means for connecting a third voltage source between said first and second electrodes for maintaining along said surfaces of said electrodes,

electrostatic potentials substantially corresponding to the electrostatic potentials for concentric spherical metal electrodes so that the extensions of the 6 electric field lines in said gap converge substantially to a point.

2. An imaging chamber as defined in claim 1 wherein said electrodes have flat parallel gap surfaces and said spaced locations are concentric with each other.

3. An imaging chamber as defined in claim 1 wherein said electrodes have concentric cylindrical gap surfaces and said spaced locations are parallel to each other.

4. An imaging chamber as defined in claim 1 wherein said means defining spaced locations comprise electrical conductors on said insulating substrates.

5. An imaging chamber as defined in claim 4 wherein said electrodes have flat parallel gap surfaces and said conductors on each electrode are concentric with each other.

6. An imaging chamber as defined inclaim 4 wherein said electrodes have concentric cylindrical gap surfaces and said conductors on each electrode are parallel to each other.

7. An imaging chamber as defined in claim 1 wherein each of said electrodes includes a dielectric sheet substrate with a plurality of spaced conductors on the gap surface thereof for connection to the corresponding voltage source.

8. An imaging chamber as defined in claim I wherein each of said electrodes includes a dielectric sheet substrate with a plurality of spaced conductors on the gap surface thereof,

additional conductors on the opposite surface thereof for connection to the corresponding voltage source, and

interconnections through the sheet between predetermined conductors.

9. An imaging chamber as defined in claim I wherein each of said electrodes includes:

a dielectric sheet substrate;

a plurality of spaced electrical conductors on the gap surface of said dielectric sheet for connection to the corresponding voltage source; and

a low conductivity layer of substantially constant resistivity and thickness on said substrate over said conductors.

' 10. An imaging chamber as defined in claim 9 wherein each of said layers has a surface resistivity in the range of about 10 to 10 ohms per square.

11. An imaging chamber as defined in claim I wherein said means defining spaced locations comprise electrical conductors at said electrode surfaces, and

each of said means for connecting said first and second voltage sources includes a voltage divider with points along the divider connected to the conductors of the corresponding electrode.

12. An imaging chamber as defined in claim 11 with said means for connecting said third voltage source providing said connection between corresponding conductors of said first and second electrodes.

[3. An imaging chamber as defined in claim I wherein each of said first and second voltage sources provides an electrode current and said third voltage source provides a gap current, with the relative magnitudes of said sources such that the electrode current is substantially greater than the gap current.

14. An imaging chamber as defined in claim 13 wherein the electrode current is at least about five times the gap current.

15. An imaging chamber as defined in claim 1 including a first resistance connected in parallel with said first electrode.

16. An imaging chamber as defined in claim including a second resistance connected in parallel with said second electrode.

17. An imaging chamber as defined in claim I wherein each of said electrode low conductivity surfaces is formed of a plurality of side-by-side sections. with adjacent sections of different and substantially uniform conductivities, with the section conductivity decreasing from the center to the edge of the electrode.

18. An imaging chamber as defined in claim 17 wherein said electrodes have flat parallel gap surfaces and said sections are concentric with each other.

19. An imaging chamber as defined in claim 17 wherein said electrodes have concentric cylindrical gap surfaces and said sections are parallel to each otherv 20. An imaging chamber as defined in claim 1 wherein said first electrode includes:

a dielectric sheet substrate;

a plurality of spaced electrical conductors on the gap surface of said dielectric sheet for connection to said first voltage source;

a first low conductivity layer of substantially constant resistivity and thickness on said substrate over said conductors; and

a second low conductivity layer of substantially constant resistivity and thickness on said first layer;

with said second layer having a resistivity greater than that of said first layer, and with said second layer having a thickness greater than that of said first layer.

21. An imaging chamber as defined in claim 20 wherein said first layer is in the order of 0.0l0 to 0.020 inches thick and said second layer is in the order of 0.040 to 0.060 inches thick.

8 22. An imaging chamber as defined in claim 20 wherein said first layer has a resistivity of about 10 to 10 ohms per square and said second layer has a resistivity of about 10 to l0 ohms per square.

23. An imaging chamber as defined in claim 20 wherein said second layer has a resistivity about at least 10 times that of said first layer and said second layer has a thickness about at least twice that of said first layer.

24. In an imaging chamber for an x-ray system, the combination of:

first and second substantially planar electrodes; means for mounting said electrodes in the chamber in spaced relation defining a gap therebetween;

each of said electrodes having an electrical insulating substrate with a low conductivity surface at said gap and a plurality of spaced conductors along said surface;

means for connecting a first voltage source to said first electrode conductors providing defined voltages therebetween;

means for connecting a second voltage source to said second electrode conductors providing defined voltages therebetween; and means for connecting a third voltage source between corresponding conductors of said first and second electrodes V for maintaining along said surfaces of said electrodes. electrostatic potentials substantially corresponding to the electrostatic potentials for concentric spherical metal electrodes so that the extensions of the electric field lines in said gap converge substantially to a point. 

1. In an imaging chamber for an x-ray system, the combination of: first and second substantially planar electrodes; means for mounting said electrodes in the chamber in spaced relation defining a gap therebetween; each of said electrodes having an electrical insulating substrate with a low conductivity surface at said gap and means defining a plurality of spaced locations along the surface; means for connecting a first voltage source to said first electrode providing defined voltages between said spaced locations of said first electrode; means for connecting a second voltage source to said second electrode providing defined voltages between said spaced locations of said second electrode; and means for connecting a third voltage source between said first and second electrodes for maintaining along said surfaces of said electrodes, electrostatic potentials substantially corresponding to the electrostatic potentials for concentric spherical metal electrodes so that the extensions of the electric field lines in said gap converge substantially to a point.
 2. An imaging chamber as defined in claim 1 wherein said electrodes have flat parallel gap surfaces and said spaced locations are concentric with each other.
 3. An imaging chamber as defined in claim 1 wherein said electrodes have concentric cylindrical gap surfaces and said spaced locations are parallel to each other.
 4. An imaging chamber as defined in claim 1 wherein said means defining spaced locations comprise electrical conductors on said insulating substrates.
 5. An imaging chamber as defined in claim 4 wherein said electrodes have flat parallel gap surfaces and said conductors on each electrode are concentric with each other.
 6. An imaging chamber as defined inclaim 4 wherein said electrodes have concentric cylindrical gap surfaces and said conductors on each electrode are parallel to each other.
 7. An imaging chamber as defined in claim 1 wherein each of said electrodes includes a dielectric sheet substrate with a plurality of spaced conductors on the gap surface thereof for connection to the corresponding voltage source.
 8. An imaging chamber as defined in claim 1 wherein each of said electrodes includes a dielectric sheet substrate with a plurality of spaced conductors on the gap surface thereof, additional conductors on the opposite surface thereof for connection to the corresponding voltage source, and interconnections through the sheet between predetermined conductors.
 9. An imaging chamber as defined in claim 1 wherein each of said electrodes includes: a dielectric sheet substrate; a plurality of spaced electrical conductors on the gap surface of said dielectric sheet for connection to the corresponding voltage source; and a low conductivity layer of substantially constant resistivity and thickness on said substrate over said conductors.
 10. An imaging chamber as defined in claim 9 wherein each of said layers has a surface resistivity in the range of about 106 to 109 ohms per square.
 11. An imaging chamber as defined in claim 1 wherein said means defining spaced locations comprise electrical conductors at said electrode surfaces, and each of said means for connecting said first and second voltage sources includes a voltage divider with points along the divider connected to the conductors of the corresponding electrode.
 12. An imaging chamber as defined in claim 11 with said means for connecting said third voltage source providing said connection between corresponding conductors of said first and second electrodes.
 13. An imaging chamber as defined in claim 1 wherein each of said first and second voltage sources provides an electrode current and said third voltage source provides a gap current, with the relative magnitudes of said sources such that the electrode current is substantially greater than the gap current.
 14. An imaging chamber as defined in claim 13 wherein the electrode current is at least about five times the gap current.
 15. An imaging chamber as defined in claim 1 including a first resistance connected in parallel with said first electrode.
 16. An imaging chamber as defined in claim 15 including a second resistance connected in parallel with said second electrode.
 17. An imaging chamber as defined in claim 1 wherein each of said electrode low conductivity surfaces is formed of a plurality of side-by-side sections, with adjacent sections of different and substantially uniform conductivities, with the section conductivity decreasing from the center to the edge of the electrode.
 18. An imaging chamber as defined in claim 17 wherein said electrodes have flat parallel gap surfaces and said sections are concentric with each other.
 19. An imaging chamber as defined in claim 17 wherein said electrodes have concentric cylindrical gap surfaces and said sections are parallel to each other.
 20. An imaging chamber as defined in claim 1 wherein said first electrode includes: a dielectric sheet substrate; a plurality of spaced electrical conductors on the gap surface of said dielectric sheet for connection to said first voltage source; a first low conductivity layer of substantially constant resistivity and thickness on said substrate over said conductors; and a second low conductivity layer of substantially constant resistivity and thickness on said first layer; with said second layer having a resistivity greater than that of said first layer, and with said second layer having a thickness greater than that of said first layer.
 21. An imaging chamber as defined in claim 20 wherein said first layer is in the order of 0.010 to 0.020 inches thick and said second layer is in the order of 0.040 to 0.060 inches thick.
 22. An imaging chamber as defined in claim 20 wherein said first layer has a resistivity of about 107 to 108 ohms per square and said second layer has a resistivity of about 108 to 109 ohms per square.
 23. An imaging chamber as defined in claim 20 wherein said second layer has a resistivity about at least 10 times that of said first layer and said second layer has a thickness about at least twice that of said first layer.
 24. In an imaging chamber for an x-ray system, the combination of: first and secoNd substantially planar electrodes; means for mounting said electrodes in the chamber in spaced relation defining a gap therebetween; each of said electrodes having an electrical insulating substrate with a low conductivity surface at said gap and a plurality of spaced conductors along said surface; means for connecting a first voltage source to said first electrode conductors providing defined voltages therebetween; means for connecting a second voltage source to said second electrode conductors providing defined voltages therebetween; and means for connecting a third voltage source between corresponding conductors of said first and second electrodes for maintaining along said surfaces of said electrodes, electrostatic potentials substantially corresponding to the electrostatic potentials for concentric spherical metal electrodes so that the extensions of the electric field lines in said gap converge substantially to a point. 